From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-181.mta0.migadu.com (out-181.mta0.migadu.com [91.218.175.181]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id C9248364055 for ; Sun, 10 May 2026 16:41:29 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.181 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778431291; cv=none; b=MPBJDk8Wm6YdFTOEJSHUn/6nxikQ6wCqTb6HVgHyrzl5P++jtEhHDmtEkCZoOlPxS7JmpRwTLLWU0+ulZdGHsFwL7iewhKOsI1NqGCH32uOeVInJpeMGCiJN1WCEbAnMPk1O0wQ/wcWjohZhVzsKh8oyUDwVGaJu41UX4Ekyu4A= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778431291; c=relaxed/simple; bh=kINXlYj0ZqFhDqV5Kh8bif5NJW8WYWpdXrgfmFjbOhA=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=QXZQ/OZ2f0//WCmRBJjfb5oqyT2udWT6T6tFIInBzORxkIYghSsSRff4MQShkUcrqFkv86h/odxDnHGqK26hBrxaOrLgfYoplg4m9J/N7vL4GOzbpx1II/nFtwRsgPvvV72wctMCO07dY+ADd2KwAt4zbXNJy0t/ieEBoFZdwb0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=VElKCbdx; arc=none smtp.client-ip=91.218.175.181 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="VElKCbdx" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1778431277; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=brqs3vlEAOl86BNBrmMrWEk4dDnUwWS00c323bfHDXU=; b=VElKCbdx9z5kUVE+NYAFBFdJeOWFfEdp/kRCiWQRA4Lq9nYosbQ3rmw89Fxd9HuExb+lpm kA1nzaEU/Dcpm8Syh8QrX03ymWUVPGUOFZQfBTvSzUXxMxorkbExwLn5bxcEJAH23LQCwA q2gkvRijxXeL7KPbWCggWoK0SNQPkGc= Date: Sun, 10 May 2026 09:41:12 -0700 Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH bpf-next v2 07/23] bpf: Add precision marking and backtracking for stack argument slots Content-Language: en-GB To: sashiko@lists.linux.dev Cc: bpf@vger.kernel.org References: <20260507213018.1128602-1-yonghong.song@linux.dev> <20260509040551.C9E2AC2BCB2@smtp.kernel.org> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Yonghong Song In-Reply-To: <20260509040551.C9E2AC2BCB2@smtp.kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On 5/9/26 6:05 AM, sashiko-bot@kernel.org wrote: > Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider: > - [High] Incorrect tracking and propagation of stack argument precision at subprogram boundaries in `backtrack_insn()`. > -- > > commit 82222fc9756a9a239fae1b810efc4aa155d2973f > Author: Yonghong Song > > bpf: Add precision marking and backtracking for stack argument slots > > This commit extends the precision marking and backtracking infrastructure to > support stack argument slots (r11-based accesses). It ensures precision > demands for scalar values passed through stack arguments are properly > tracked. > >> diff --git a/kernel/bpf/backtrack.c b/kernel/bpf/backtrack.c >> index 93738a076c888..17f254a02831f 100644 >> --- a/kernel/bpf/backtrack.c >> +++ b/kernel/bpf/backtrack.c > [ ... ] >> @@ -336,6 +351,19 @@ static int backtrack_insn(struct bpf_verifier_env *env, int idx, int subseq_idx, >> return 0; >> bt_clear_reg(bt, load_reg); >> >> + if (hist && hist->flags & INSN_F_STACK_ARG_ACCESS) { >> + spi = hist->spi; >> + /* >> + * Stack arg read: callee reads from r11+off, but >> + * the data lives in the caller's stack_arg_regs. >> + * Set the mask in the caller frame so precision >> + * is marked in the caller's slot at the callee >> + * entry checkpoint. >> + */ >> + bt_set_frame_stack_arg_slot(bt, bt->frame - 1, spi); >> + return 0; >> + } >> + > When backtracking a callee's read of an incoming stack argument via BPF_LDX, > the logic correctly sets the mask in the caller frame (bt->frame - 1). > > [ ... ] >> @@ -426,6 +465,18 @@ static int backtrack_insn(struct bpf_verifier_env *env, int idx, int subseq_idx, >> bpf_bt_set_frame_reg(bt, bt->frame - 1, i); >> } >> } >> + /* >> + * propagate callee's incoming stack arg precision >> + * to caller's outgoing stack arg slots >> + */ >> + if (bt_stack_arg_mask(bt)) { >> + for (i = 0; i < MAX_BPF_FUNC_ARGS - MAX_BPF_FUNC_REG_ARGS; i++) { >> + if (!bt_is_frame_stack_arg_slot_set(bt, bt->frame, i)) >> + continue; >> + bt_clear_frame_stack_arg_slot(bt, bt->frame, i); >> + bt_set_frame_stack_arg_slot(bt, bt->frame - 1, i); >> + } >> + } > Does this propagation logic operate on the correct mask? > > Since the callee's BPF_LDX instructions already marked the incoming stack > arguments directly in the caller's frame (bt->frame - 1), > bt_stack_arg_mask(bt) for the current frame (bt->frame) shouldn't contain > any incoming stack arguments here. You are right here. bt_is_frame_stack_arg_slot_set(bt, bt->frame, i) should be 0 so the above whole loop is not necessary. > > If bt_stack_arg_mask(bt) is non-zero at this point, wouldn't that indicate > leftover uninitialized outgoing stack arguments from the callee that were > never matched by a BPF_STX? > > Instead of propagating these leftovers to the caller frame, should this > condition be treated as a verifier_bug(), similar to how leftover stack > slots are handled just above this code? Yes, verifier_bug() makes sense to capture potential issues. This will be similar to potential bugs with register spills. >